Vacuum tube amplifier circuits for coded carrier current



Dec. 15, 1953 L. R. ALLISON ,6 3

VACUUM TUBE AMPLIFIER CIRCUITS FOR CODED CARRIER CURRENT Filed Feb. 8,1949 2 Sheets-Sheet 2 I 2 MP IN V EN TOR.

ATTORNEY Patented Dec. 15, 1953 UN ITED STATES ENT' QE'FI fiE o2,662,984. VACUUMKMPLIFIER GIREUITB FGW GDDED GARRIEE GURRENT Leslie. Rm@llison Forest nun-1a., assignor. to

Westinghouse Brake Commune. a corporat tion otlennsyl'vaniaAnplication'lwimm 8, 1e4a,-,-senuno:.1a;m:

My: invention. relates to vacuumu tube. amplk fier circuits, and moreparticularly to vacuum tube. amplifier circuits for: use with r coded:carrier current.

There are many; signaling: systems: which? use coded carrier:currentand: for which rsystemsiexiacting; safety requirements are:preseribedi be cause theoperationmust be asznearlyonehum dred percentsafe as can be -reasonably obtained. Als0,-. a: failure: ofi thesystemif: it does occur should been the-side of safety-.. example;railway cab I signal systems in: many: cases-x. use alternating; currentwhich. iscoded by being priodicallyinterrupted Ordinariiy-"the-altermning current is1of a1specialifrequency such cas:100 cycles: per second:andwthe current is coded: at any one-of. several. differentcodeerateseachlof which. code rates? reflects at designated; tra-fiiccondition; In: these: railway cab: signal: system: the track rails are.arranged. in. track: sections and each traclcsection: is provided with atrack circuit that includes means for: supplying coded alternating;current. to; the: rails? of. theisection; Inductors-are mounted on:thetrain for-inductive relation to the-rails to: pick: up 1 anelectromotive force in response: to the -codedi alternating; our-.--

rent flowing in the rails-5 This. electrom'otive force is usedto operatera train-carried-codefo'l lowing relay; which in turn governs-,thewabsige nals and other train controhdevices; The-elecitromotiveforcewpicked up'by the'inductorsis of a low energy level-andianamplifieris .iintertposed. between theinductors andathe.codeefol-v lowing'relay.-

Due to thesafety requirements .set unfonthese railway cab signalsystems;the amplifier circuits must avoid. false. operations; due to; circuitfail+ ures or: due: to; failure. of.- energy. Inllineiwith the erequirements; the amplifier circuitsusually includezan input filter.tuned to resonance attthe frequency of the alternating; current used sothat the. apparatus will. beimmune: toextraneous energy picked upbytheinductors. due: torthe rails carrying. propulsion current. andstray a1 ternating currentsfrom commercial powerlines and other:sources. circuits are arranged, insofar. as it-is possible, so that abroken. ormisplaced circuitl element will not result in a falseoperation of" the apparatusto produce-a false proceedsignalindn cation.Since. an open; circuit condition" may create an intermittent opening of"the circuitidue to. the vibration of the train, theamphfiercir cuitsshould protect against" an" improper operation of thecodefollowingrelaydue teen-interrmittent-opening of any of" ttiewii'cuitsi 1 Furthermore,the amplifier Again in zamplifiercircuitsof. the? type here contemplated8'.- givenbias-.- voltage for: the vacuum tube ispreferably used andthe. safetyrequirements. mentioned above makeit 1 necessary toiassurethat-ia loss-of:theybiasrvoltage doesnot result: in o. falseoperatiomofithe apparatus In View of.- the problems encountered due' to theisafetyyrequirements. for. signaling, systemsrof the typehere involvedaprincipal: object'- of: my invention .is the provision of vacuum tube:ampli+ fien circuits-havingimprovedsa-fety features.

Another object of. my. invention. is: theprovisionm'vacuum*tubeamplifiertcircuitsincorporat ing novel meanstoieliminate:falseoperation'due to an r intermittent open circuitcondition."

Again an: objectof invention is the: provision; of... vacuumi tubamplifien circuitsincorporating a novel and improved automatic' biascontrol means;

(ether-i featuresiobjectsanm advantages: of:- my invention. will. appearas 1 the. specification. progresses.

In practicing my invention I. provide a vaccum-tube amplifier circuitnetwork-that includes input filter. Thisifilten preferably includes atransformer having, tuned primary. and secondary windings, pthe primary;winding-beingadapted to.- receivev tha signaling energy and the:secondary: winding being. connected 1 toa..- controL elec-' trodeof-theviacuumtube. The outputofthisamplifier circuit network includes acoupling-trans.- former,,the-primarywindingmf which isincluded in theanodecircuihotthe vacuumtube andthe secondary winding.- ofwhich-transformer. is: connected to. encode following relay.-Thus-,ta-sharp rise or change inttie valuerofthe anode current willinducetan impulseof 'elect'romotive forcein theisecondary winding: forenergizing. the relay; the impulse being. of.- one polarity when theanode :currenhrises and being of-itlie reverse. p.0- lari ty.whernthe-anodecurrent :dec-reases-sliarply-- Ti'iemode following, relay,is a polar" relay.- having its .cont'acttmember.operatedto a first andesec end position ihmesponsetoenergizing impulses ofioppositoipolaritysuppliedito the-relay:

L'provideamacuum .tubethat is constructed .in s uclia manner that-eachend of its con-trollel'ec trade-or gridiisb'rough't outttoxan externalbase pin or terminal'so.-that a circuit. may bearranged to have thecontrollgrid elementLandiits two tenmum-1s in series therewith. Thatisto say; the vacuum: tube isprovided with two external terminalswn'i'cn' are'connectedto'. the oppositeends'.

or at least th two spaced points or the control grid. Them-critic)gridand its twc-terrmnaisin senes are'in'eiud'eu' iir a circuit "which"comprises resistive or electrical conductive elements and capacitiveelements, the resistive elements being connected to one of the gridelements and the capacitive elements being connected to the otherterminal of the control grid. Preferably the secondary winding of thefilter transformer is made at least one of the conductive elements ofthis circuit and a capacitor of the filter is preferably mad at leastone of the capactitive elements of the circuit. This circuit includingthe control rid and its two terminals is connected to the cathode of thetube to form a control grid cathode circuit, a bias voltage source beinginterposed in the circuit adjacent the cathode. This bias voltage sourceis preferably poled to bias the control grid negative in potential withrespect to the cathode by a voltage sufficient to give substantiallyzero anode current when no signaling energy is received. This controlgrid cathode circuit is connected to the two terminals of the controlgrid in such a manner that no portion of the circuit can become openwithout either the negative bias voltage of the control grid beingmaintained subsequent to the open circuit condition or the bias voltagebeing reduced at such a gradual rate subsequent to the open circuit thatthe anode current builds up so slowly that any impulse induced in thecoupling transformer as 'a result of this building up of the anodecurrent is insufiicient to operate the code following relay. In this wayan intermittent open circuit in any point of the circuit network wouldnot result in energy impulses being passed to the code following relayfor operation thereof.

I shall describe several forms or" vacuum tube amplifier circuitsembodyin my invention and shall then point out the novel featuresthereof in claims.

In the accompanying drawings, Figs. 1, 2, 3, 4, and 5 are diagrammaticviews showing five different forms of vacuum tube amplifier circuits,each of which forms embodies my invention.

In the drawings the amplifier circuits are illustrated as being usedwith a railway cab signal system, but it is to be understood that whilethe vacuum tube amplifier circuits provided by this invention arepeculiarly adaptable for use in railway cab signal systems, the circuitsare not limited to this one use and there are many other places wherethe circuits can be used to an advantage.

In each of the several views like reference characters are used todesignate similar parts.

Wherever the term vacuum tube is used in the specification and claims,it is understood to mean a device consisting of an evacuated eno closurecontaining a number of electrodes between two or more of whichconduction of electricity through the vacuum or contained gas may takeplace. That is, the term vacuum tube is here used to cover an electrontube or a gas tube.

Referring to Fig. 1, the reference characters la and lb designate thetrack rails of a railway and which rails are formed in the usual mannerinto track sections. The rails of each section are included in a trackcircuit having a source of alternating current connected across therails at the exit end of the section. The alternating current is of adesignated frequency and is coded at any one of a plurality of diiierentcode rates according to different traflic conditions. The trackwayapparatus for supplying the coded current to the rails la and lb is notshown since its specific structure forms no part of my invention andthere are several well-known arrangements that can be used. For example,the trackway apparatus may be similar to that disclosed in LettersPatent of the United States No. 1,986,679, granted January 1, 1935, toLloyd V. Lewis, for Railway Traffic Controlling Apparatus. As an aid inthe understanding of my invention the alternating current supplied tothe rails la and lb will be assumed to have a frequency of cycles persecond and as being coded at 180, 120, and 75 interruptions per minuteto reflect clear, approach medium, and approach traffic conditions,respectively. The absence of rail current and the presence of non-codedrail current refleet a stop or slow speed trafiic condition. Thus eachcode is made up of alternate on periods during which current flows inthe rails and off periods during which no current flows in the rails. Itwill be understood, however, that my invention is not limited to theabove assumed frequency for the alternating current and the abovementioned code rates for the coding thereof.

The train carried apparatus of Fig. 1 includes an inductor IN and anamplifying unit AM.

The inductor unit IN includes two windings I l and I2 which are mountedon the train in inductive relationship with the rails la and lb,respectively. Thus an electromotive force is induced in the windings I land I2 due to the coded alternating current supplied to the rails in themanner described above. The windings ll and I2 are connected to addtheir electromotive forces when current flows in opposite directions inthe two track rails la and lb at any given instance. Consequently theinductors ll and I2 and the trackway apparatus associated therewithconstitute a source of coded carrier current. The windings ll and 12 ofthe inductor IN are connected by wires I3 and I4 to input terminals TCand FT of the amplifier unit AM.

The amplifying unit AM comprises a filter Fl, a vacuum tube VT, acoupling or master transformer MT, and a code following or master relayMR. The filter Fl comprises capacitors Cl and C2 and a transformer Tlhaving independent primary and secondary windings l5 and iii,respectively, but an autotransformer may be used. The primary winding l5of transformer Tl and the capacitor CI in series are connected acrossthe terminals TC and FT, the parts being proportioned for this circuitto be tuned to resonance at the frequency of the carrier of thesignaling current, which in the case here used for illustration is acurrent of 100 cycles per second.

The secondary winding l6 and the capacitor C2 are included in the filterFl in a manner to be more fully discussed hereinafter.

The vacuum tube VT is preferably a high vacuum indirectly heated cathodetube but other types of tubes may be used. As disclosed, the tube VT isprovided with a filament or heater H, a cathode l8, an anode or plateIt, a screen grid 20, and a control grid 2l. The tube VT is ofconventional construction except the wire or element forming the controlgrid El has both ends thereof brought out to separate base or terminalpins which are indicated by the numerals l and 5. This constructionpermits a circuit to be established through the tube with the controlgrid 2l in series therewith. This construction of the control grid 2lmay be accomplished in any suitable manner. For example, it may beaccomplished by mounting the wire forming the control grid on aninsulating member and connecting the two ends of the wire 5 to terminalpins 1! and :5. The Lremaming zelemerits :of "the tube VT .are broughttout 'fto :base pins in the :usual construction, the :filament I? beingconnected to :base .pins .2 :and =1, ithe cathode it to abase .pin :8,the iscreenegri'd :ZIlto'a base pin and theanode I 9'to :a sbaseipin1-3.

The tube VT designed .for operation U11 a single volt source of directcurrent, it being contemplated that the usual trainili htingageneratoror batterywillserve asasource of energy for the amplifyingunitrAM.'In.the:drawings,2the source of energy for'the amplifying unit:indicated by the positive terminal BBQrandethe negative terminal C. Itis to be understood, however, that the tube maybe designed to use -a.power source of some Other voltage'and if desired the filament may beheated'from a low voltage source'and the anodeandscreen grid excitedfrom a high voltage'source. The filamentor heater I! of tube'VT isconnected directly acrcss the terminals B32 and C, as will be apparentby an'inspection of Fig. l and the tube is in an active condition. Tworesistors RI andRZ in'series are also connected across the terminals-332 and C to form-a voltage divider from which a bias vvoltage isobtained, as will appear shortly.

An anode circuit is formed for the tube VT by the anode It beingconnected to terminal 332 through a winding 22 of the coupling or'mastertransiormer'MT and the cathode iii of the tube being connected 'to thejunction terminal of the resistors Ri and R2. The screengrid is alsoconnected to the positive terminal B32 of the power source.

The control grid '2! and itstwo'terminals I and 5 are included in acircuit that is connected to the secondary winding I5 of the filtertransformer Ti, and the capacitor C2. This circuit extends from the topterminal of winding It as viewed in Fig. 1 to the terminal I, controlgrid 3:, terminal 5, capacitor C2 and thence to the lower terminal ofthe secondary winding I6. This circuit including winding 16, controlgrid and capacitor C2 is tuned to resonance at the carrier frequency ofthe signaling current and the two tuned circuits one including"theprimary winding Id 'of transformer TI and the capacitor Cl and theother including the secondary winding it of the transformer and thecapacitor C2, form the input filter PI. The control grid ii isconnected'to the cathode l8 through one ath which extends from terminal"I through winding is of the transformer TI and resistor Bi and itfollows that the control grid is biased negative in potential withrespect to the oathode bya voltage equal to the voltage drop across theresistor RI, the resistor RI forming a source of bias voltage.Furthermore, the terminal '5 of the control grid 2| is connected .to thecathode through capacitor C2 and the resistor RI. The parts are soproportioned that the negative bias voltage applied to the control gridfrom the resister RI is sufiicient to bias the tube to substantially azero anode current, thisbeing the preferred arrangement although a biasof a dinerent value may be provided.

The winding 22 of the coupling transformer MT is provided with a by-passcapacitor '23 and a secondary winding 24 of the "transformer isconnected to the operating winding of the code relay MR. Relay MR ispreferably-astick polar relay operable in response to a predeterminedvalue of energization. Therelay MR is provided with .a contact member 25which is operated in one direction to a first position when acurrentimpulse "of :one :polarity is supplied to the relay winding :and themember .25 is operated in :the other :directionstoa second position whenthe energizingimpulse isof the opposite polarity. The contactmemberi25is usedto govern decoding and signalingrmeans of anyo'f thewell-known arrangements, and twhichrequipment is not shown for'thesakeof 'simplicityzsince' it forms no part-cf my presentiinvention andits 'showingis not required for a full understandingthereof. Thedecoding and signaling apparatus may be similar tozthat fdisclosedin mycopending application for Letters Patent of the United States, I SerialNo. 7.40;3l1 ,filed Aprile, 194'], for Train Carried Cab SignalApparatus, tnow Patent No. 2 3 62A'54, granted February 22, 1949. It issufiicient for the present:applic'ation to point outithat code-operationofthetrelayMR'at' the 180, 120, and IE-code rates eife'ctsra clear,approach:medium,-and approa;ch x'cab :signal indication, respectively.Also, \when :relay .MR :is deenergized and is :not operated'a stoporslow.speed" cab signal is .efiected.

:In describing the-operation of the apparatusof 'Fig. 21, :Is'hallifirst consider that the tube VT is heatedand that .no signalingcurrent is being supplied to the rails Ia and it. Under this con-.dition atheibias voltage derived :from resistor "RI for the controlgridl I efiects a substantially zero anode current. There being novariations in the anode 'currentfrom..its zero'value, therewill be noelectromotive .force induced in the second- 'ary windinglfi'of themastertransformer MT and thezrelay MR'is deenergized with the result that itscontact member25 remains atthe position'to .whichrit was :last moved.This non-operation of the .relay MR creates the"stop or slow speed cabsignal. Ishall nextassume that alternating current coded at the 180 coderate is supplied to the rails Ia and I b and acorrespondingelectromotive force is induced in the windings I I and 1-2of the inductor. This induced electromotive force is applied to theterminals TC and F1 of the amplifying unit AMand a corresponding'electromotive force is induced in secondary Winding It. Theelectromotive force thus induced in the secondary winding I6 is appliedto the control grid circuit of the tube VT. Each positive half cycle ofthe electromotive force thus applied to the control grid 2| drives thegrid 2| in the positive direction in opposition to the fixed.biasvoltage derived from resistor RI and a current impulseflows in theanode circuit of the tube. Thus there is an increase in the averagevalue of the anode current during each on code period of ,the codecarrier current and the anode current decreases to substantially zeroduring-each off code period. The carrier variations of the anode currentare by-passed by .capacitor'23, but the code variations in the value ofthe :anode current create corresponding impulses insecondary winding 24,the impulse being of onepolarity when the current increases during theon code period, and being of the opposite polarity when the currentdecreases during the oil code period. These impulses induced insecondary winding 2d are applied to the relay MR and the relay MRis'operated at a rate corresponding to the 180 code rate of the railcurrent and in turn the relay effects a corresponding clear cab signal.

The operation of the apparatus of Fig. 1 when current'of either the or75 code rate is supplied to the rails is the same as abovedescribedforcurrent of coderateexcept that the relay MR is operated at ratescorresponding to the 120 and '75 code rates of the rail current andcorresponding cab signal conditions are established.

It is apparent that electromotive forces picked up by the inductor INdue to alternating current of a frequency other than 100 cycles persecond will be substantially suppressed due to the input filter Fl. Inother Words, the amplifying unit is substantially immune to currentsother than the 100 cycle signaling current.

I shall next consider open circuit conditions for the apparatus of Fig.1 and the protection provided against false operation of the relay MRdue to an open circuit condition when no signaling current is suppliedto the rails. In the first place the screen grid 20 which is connectedto the positive terminal B32, effects a higher degree of sensitivity forthe tube than would prevail if a lower voltage is applied to the screengrid. Thus, an open circuit in the connection of the screen grid 20results in a lower sensitivity for the amplifier tube and if there is afailure it will be that not enough energy is supplied to the relay MRfor operation thereof in response to the rail current. With the relay MRnot operated the stop or slow speed signal is created and which would bea more restrictive signal. That is, the failure, if any, due to an opencircuit in the screen grid connection would be on the side of safety.

Again, it is clear that any open circuit in the anode circuit of thetube when no signaling en'- ergy is supplied to the rails will cause noelectromotive force to be induced in winding 24 for energizing the relayMR because the anode current is normally of zero value.

Considering the effects resulting from an open circuit in the circuitassociated with the control grid of the tube, the series arrangement ofthe secondary winding I6, control grid 2i and capacitor C2, assures thatthe circuit cannot open at any point without leaving either thesecondary winding (6 or the capacitor C2 connected between the controlgrid and the cathode. If the capacitor portion of this circuit is open,the bias of the grid i maintained through the conductive path includingsecondary Winding l6 and the anode current is maintained at its zerovalue. Should the conductive or secondary winding [6 portion of thecircuit become open, the capacitor C2 remains connected between thecontrol grid 2! and the cathode i8. At the instant the secondary windingopens, the capacitor C2 begins to build up a charge, the final chargebeing equal to the bias potential across the resistor RI. The path ofthe charging current is from the positive terminal of resistor RIthrough cathode I8, the high resistance tube space between the cathodel3 and the control grid 2|, and the capacitor C2 to the negativeterminal of the resistor RI. The drop in potential between the cathodeand the control grid clue to this charging current helps to maintain thenegative potential of the grid until the capacitor C2 charges to thefull bias potential. Due to the high resistance between the cathode IEand the control grid 2| within the tube, this charge on the capacitor C2builds up relatively slowly and the building up of the anode current dueto the loss of the bia voltage on the control grid is correspondinglyslow with the result that little or no electromotive force is duced inthe secondary winding 24 of the transformer MT due to the change in theanode current. Thus, there is also no operation of the relay MR due toan open circuit in the secondary winding portion of the grid circuit. Itshould be noted that since the entire length of the control grid 21forms a part of the circuit associated with the control grid, a break inthe grid within the tube is checked just as much as a break in thecircuit outside of the tube. It follow from the foregoing description ofthe operation of the apparatus of Fig. 1 that the amplifier circuits areimmune to extraneous current, and a break or intermittent break in anycircuit element does not result in an operation of the relay MR.

In Fig. 2, the amplifier circuits are the same as in Fig. 1 except forthe control grid circuit and only thi one circuit of Fig. 2 needs to bedescribed. In Fig. 2 the capacitor C2 is connected directly across thesecondary winding I5 of the filter transformer. The control grid 2| inseries with a resistor R3 and a capacitor C4 are connected across thetuned circuit comprising the secondary winding 16 and capacitor C2, thisgrid circuit extending from the top terminal of the secondary winding I6through capacitor C4, terminal l of the tube, control grid 2i, terminal5 of the tube and resistor R3 to the lower terminal of the secondarywinding III. The source of bias voltage, the resistor RI, has itspositive terminal connected to the cathode I8 and its negative terminalconnected to the lower terminal of the secondary winding i6 and whichterminal is also common for one terminal of capacitor C2 and oneterminal of resistor R3. Thus, in Fig. 2, the control grid 2| itsterminal I connected to the cathode through the capacitor C4, winding l5and the bias voltage source RI, and has its other terminal 5 connectedto the cathode through resistor R3 and the bias voltage source.

The operation of the circuits for Fig. 2 is substantially the same athat obtained for the circuits of Fig. 1 and only the operationresulting from an open circuit condition in the control grid circuitneeds to be considered. Should an open circuit condition occur in theportion of the control grid circuit including capacitor C4 and windingI5, the control grid is still connected to the cathode through the biasvoltage source RI and the resistor R3 and the negative bias voltage ofthe control grid is maintained with the result the anode current of thetube is held at its zero value. If the open circuit occur in theresistance portion of the circuit, that is, in the path includingresistor R3, then a charge is slowly built up on the capacitor C4 due tothe voltage across resistor RI, the charging current flowing from thepositive terminal of resistor RI through cathode I8, tube space tocontrol grid 2I, capacitor C4 and winding I6 to the negative terminal ofresistor Ri. The potential drop between the cathode and the control griddue to the charging current helps to maintain the bias of the controlgrid until the capacitor C4 is charged to the voltage drop across theresistor RI. Thus, the loss of the bias on the control grid and thecorresponding building up of the anode current are at a relatively slowrate and consequently no electromotive force or a very smallelectromotive force is induced in secondary winding 24 of thetransformer MT and this small electromotive force is insufficient tooperate the relay MR.

In Fig. the circuits are the same as in Fig. 2 except the control gridcircuit for the tube is modified by the resistor R3 being connectedbetween the top terminal of the secondary winding E8 of the filtertransformer and the terminal 5 of the tube. In this arrangement of Fig.3, an open circuit in capacitor C4 leaves the negative bias voltage forthe control grid 2| at it normal .a-ecaasa value. due. to. the. path.throughthe winding I6 and resistor R3to the terminal 5of'the tube. Inthe case of" an"open:circuit' conditionin the resistor R3, the negativebias is. retained until the capacitor C6 is charged by the voltagev dropof resistor RI through the tubespace between the cathode and controlgrid andthrough the sec ondary; winding Igor throughzthe capacitDr CZ.Thus. in the circuits of" Fig. 3'a.false operation ftherelay MR due toany, intermittent. open circuit condition of the circuitsassociate'd'with the tube is avoided.

In Fig. 4, the circuits are modified to include an asymmetric unit RX.The tuned circuit comprising the secondary winding I6 of transformer TIand capacitor C2 has its top terminal connected through asymmetric unitRX to grid terminal I of tube VT and the other grid terminal isconnected through resistor R3 to the lower terminal of the tunedsecondary winding I6. A capacitor C4 is connected across the tunedwinding I6 and the asymmetric unit RX in series. The unit RX may be anyone of several known forms of asymmetric units. For example, it may be acopper oxide rectifier unit. In Fig. 4 if an open circuit occurs in thesecondary winding I6 portion of the control grid circuit, the negativebias is maintained on the control grid through the resistor R3. If anopen circuit occurs in the resistor R3 portion of the circuit, thenegative bias is maintained during the charging of the capacitor C4 dueto the voltage drop of resistor RI applied through the tube spacebetween the cathode and the control grid. This charge of the capacitorC4 builds up slowly due to the relatively high resistance of the tubespace and the corresponding building up of the anode current due to theloss of the bias voltage on the control grid is so gradual that anyelectromotive force induced in secondary winding 24 of the transformerMT is insuflicient to operate the relay MR. The asymmetric unit RX ispoled to pass the half cycles of the signaling current that tend todrive the control grid in the positive direction.

In Fig. 5, the circuits of Fig. l are modified by the capacitor C2having one terminal connected directly to the cathode I 8 instead of tothe negative terminal of the resistor RI as shown in Fig. 1. With thearrangement of Fig. 5, the capacitor C2 is normally charged by the biasvoltage across resistor RI due to the circuit through the control grid2| and winding I6. If an open circuit occurs in winding I6 of thecontrol grid circuit of Fig. 5, the negative bias of the con trol grid2! is slowly reduced due to the slow discharge of the capacitor C2through the grid cathode conductance of the tube. Thus the building upof the anode current in this case is slow and any resultantelectromotive force induced in the secondary winding 24 of transformerMT is insufficient to operate the relay MR. Also, an open circuitcondition on the capacitor side of the control grid circuit of Fig. 5leaves the negative bias of the control grid maintained through thesecondary winding l6 and no variation of the anode circuit current takesplace.

I have found that in the circuit arrangement of Fig. 5, the usual valueof the resistor RI is too small to appreciably affect the operation ofthe apparatus.

Thus the protection provided by the circuit arrangement of Fig. 5against false operation of the relay MR due to intermittent open circuitcondi tions of the circuits is substantially the same as provided by thecircuits of Fig. 1.

Although I have herein shown and described 10 several: forms: of vacuumtube amplifier circuits embodying; my invention, it' is to be understoodthat various: changes and modifications may be made therein within thescope of the'appended claims Without departing from the spirit and scopeof myinvention.

Having; thus described: my invention, what' I claim is-:

l;.Ii1 combination; a vacuum tube having an anode; a: cathode and acontrol grid; said tube prov'ide'd with: two external terminals oneconnected to each end of said grid, a transformer having a primary and asecondary winding, an anode circuit including a power source and saidprimary winding connected to said anode and cathode to produce a currentflow in the anode circuit as governed by voltages applied to saidcontrol grid, a signaling source of coded carrier energy, anotherwinding to which said energy from said signaling source is supplied attimes and is not supplied at other times, a capacitor, a direct voltagebias source; a grid circuit including in series said another winding,said grid and its two terminals and said capacitor; said bias voltagesource having its positive terminal connected to said cathode and itsnegative terminal connected to said grid circuit at the junction of saidcapacitor and said another winding to bias the control grid negative involtage with respect to said cathode and at a voltage to produce asubstantially zero anode circuit current when no energy from said sourceis supplied to said another winding, and a polar code following relayconnected to said secondary winding to be operated by the electromotiveforces induced in said secondary winding in response to variations ofthe anode circuit current due to the signaling energy received by saidanother winding, said control grid being checked by said series gridcircuit and said bias voltage being effectively maintained by the biasvoltage source when an open circuit of said grid circuit occurs, wherebyvariations of said anode circuit current and false operation of saidrelay due to an intermittent open circuit of the tube circuits when nocoded carrier is supplied to said another winding is avoided.

2. In vacuum tube amplifier circuits; a Vacuum tube provided with ananode, a cathode and a control grid and having two external terminalswhich are connected to spaced points of the control grid; a couplingtransformer having a primary and a secondary winding, a power source, ananode circuit including said primary winding and said power sourceconnected across said anode and cathode for producing a current flow inthe anode circuit variable according to code variations of voltagesapplied to said control grid, another winding adapted to receive codedalternating current, a direct voltage bias source, a capacitor, saidcapacitor connected across said another winding through said controlgrid and its two terminals in series to form a control grid circuit,said control grid circuit being tuned to resonance at the frequency ofsaid alternating current, said source of bias voltage connected to saidcathode and to said reconant circuit at the junction of said anotherwinding and capacitor and poled to bias said control grid negative inpotential with respect to said cathode to produce a substantially zeronormal anode current when no voltage is applied to said control grid, asource of coded alternating current effectively coupled at times to saidanother winding, a code responsive device connected to said secondarywinding of said transformer and code operated in response to codevariations of said anode circuit 11 current due to said codedalternating current received by said another winding, and said controlgrid being checked by said series control grid circuit and said biaspotential being retained through said another winding when an opencircuit occurs in said capacitor and the loss of said bias potentialbeing delayed due to the charging of the capacitor by said bias voltagesource through the cathode to grid tube space when an open circuitoccurs in said another winding.

LESLIE R. ALLISON.

References Cited in the flle 01 this patent UNITED STATES PATENTS NumberName Date Wright Oct. 23, 1917 Roberts May 20, 1930 Strutt et al Aug.18, 1942 Allison Dec. 14, 1943 Volz July 18, 1950 Martin Feb. 13, 1951

